Polyester copolymer fiber having enhanced strength and dyeability properties

ABSTRACT

The invention is a method of producing a polyester filament which has a superior combination of tensile, dyeability and shrinkage properties. The method comprises forming a polyester copolymer from a mixture consisting essentially of terephthalic acid (TA) or dimethyl terephthalate (DMT), ethylene glycol, adipic acid, and pentaerythritol wherein the adipic acid is added in an amount of between 1.3 and 3.2 weight percent of the terephthalic acid and pentaerythritol is added in the amount from 175 to 700 ppm by weight of terephthatlic acid; forming the filament from the copolymer, drawing the copolymer filament, and heat-setting the drawn filament. The invention also comprises the enhanced fiber formed by the process having an atmospheric dyeability greater than that of an unenhanced PET produced under identical conditions except for the addition of the adipic acid and pentaerythritol, a modulus of greater than 3.0 g/denier, a tenacity from about 5 to 7 g/denier and a hot air shrinkage of less than 10%.

FIELD OF THE INVENTION

The present invention relates to the manufacture of polyester fibers fortextile applications, and in particular relates to an enhanced polyestercopolymer fiber material which demonstrates a combination of improvedtensile properties and improved dyeability.

BACKGROUND OF THE INVENTION

Polyester has long been recognized as a desirable material for textileapplications. The basic processes for the manufacture of polyester arerelatively well known and straight forward, and fibers from polyestercan be appropriately woven or knitted to form textile fabric. Polyesterfibers can be blended with other fibers such as wool or cotton toproduce fabrics which have the enhanced strength, durability and memoryaspects of polyester, while retaining many of the desired qualities ofthe natural fiber with which the polyester is blended.

As with any fiber, the particular polyester fiber from which any givenfabric is formed must have properties suitable for manufacture,finishing, and end use of that fabric. Typical applications include bothring and open-end spinning, either with or without a blended naturalfiber, weaving or knitting, dyeing and finishing. In addition, it haslong been known that synthetic fibers such as polyester which areinitially formed as extruded linear filaments will exhibit more of theproperties of natural fibers such as wool or cotton if they are treatedin some manner which changes the linear filament into some other shape.Such treatments are generally referred to in the art as texturizing, andcan include false twisting, crimping and certain chemical treatments.

In a homopolymeric state, polyester exhibits good strengthcharacteristics. Typical measured strength characteristics includetenacity, which is generally expressed as the grams per denier requiredto break a filament, and the modulus, which refers to the filamentstrength at a specified elongation ("SASE"). Tenacity and modulus arealso referred to together as the tensile characteristics or "tensiles"of a given fiber. In relatively pure homopolymeric polyester, thetenacity will generally range from about 4.5 to about 7 g/denier.

In many applications, of course, it is desirable that the textile fabricbe available in a variety of colors, accomplished by a dyeing step.Substantially pure polyester, however, is not as dyeable as most naturalfibers, or as would otherwise be desired, and therefore must usually bedyed under conditions of high temperature, high pressure, or both, or atatmospheric conditions with or without the use of swelling agents,commonly referred to as "carriers". Accordingly, various techniques havebeen developed for enhancing the dyeability of polyester.

One technique for enhancing the dyeability of polyester is the additionof various functional groups to the polymer to which dye molecules orparticles such as pigments attach more readily, either chemically orphysically, depending upon the type of dyeing technique employed. Commontypes of additives include molecules with functional groups that tend tobe more receptive to chemical reaction with dye molecules thanpolyester. These often include carboxylic acids (particularlydicarboxylic or other multifunctional acids), and organometalic sulfateor sulfonate compounds.

It is known in the art that adipic acid can be added to terephthlaticacid (TA) or dimethyl terephthalate (DMT) to produce a polyester withimproved dyeability properties of the fibers. Adding increased amountsof adipic acid during production of polyester will increase thedyeability of the resulting fiber. However, there are a number ofdisadvantages associated with the addition of adipic acid for thepurposes of enhancing dyeability. These disadvantages are shown in theloss of strength of the fiber as indicated by the lower modulus andtenacity measurement of the fiber.

It is also known in the art that pentaerythritol at low levels of lessthan about 450 ppm by weight of the TA or DMT can be incorporated intopolyester for providing improved strength of the fiber including themodulus and tenacity of the fiber. However, at levels of greater than450 ppm, the pentaerythritol may result in decreased strength anddyeability of the fiber. U.S. Pat. No. 4,113,704 to MacLean et aldiscloses use of pentaerythritol as a branching agent to enhancedyeability within a limited range and also to enhance the productivityof polyester.

It is also known that polyethylene glycol (PEG) can offer variousadvantages when incorporated into polyester for textile fibers,including improved dyeing characteristics. Nevertheless, there are anumber of disadvantages associated with the application of PEG topolyester as in the case of adding adipic acid to the polyester, inparticular, the diminished strength of the fiber.

Accordingly, there remains a need to develop a suitable additive forpolyester fibers that enhance the dyeing properties of the polyesterfiber while also enhancing such characteristics as strength, hot airshrinkage (HAS), and dyeability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof producing a polyester filament which has a superior combination ofdyeability, tensile and hot air shrinkage properties. The methodcomprises forming a polyester copolymer from a mixture consistingessentially of terephthalic acid (TA) or dimethyl terephthalate (DMT),ethylene glycol, adipic acid, and pentaerythritol and optionally asolution of adipic acid, ethylene glycol, and bishydroxyethyladipate(BHEA), which is the product of adipic acid and ethylene glycol. Theadipic acid is added in the amount of 1.3 to 3.1 weight percent based onTA or DMT and the pentaerythritol is added in the amount of betweenabout 175 and about 700 ppm based on TA or DMT. The copolymer is drawninto filament at a draw ratio and temperature sufficient to produce thedesired enhanced tensile properties in the filament, after which thedrawn filament is heated at a temperature sufficiently high enough toset the desired enhanced tensile properties in the copolymer filamentand to maintain the shrinkage of the copolymer filament substantiallythe same as the shrinkage of the nonenhanced polymer filament, butwithout lowering the dyeability of the resulting fiber below thedyeability of the nonenhanced fiber.

Because of the relationship between tensile strength and dyeability, theinvention also provides a method of enhancing the dyeability ofpolyester fiber while maintaining the tensiles of that fibersubstantially equivalent to its tensile strength when not enhanced. In asimilar manner, the invention provides a method of concurrentlyenhancing both dyeability and tensile strength, as well as maintainingthe hot air shrinkage compared to a nonenhanced polyester fiber or apolyester fiber enhanced with either one of the adipic acid orpentaerythritol.

The foregoing and other objects, advantages and features of theinvention, and the manner in which the same are accomplished, willbecome more readily apparent upon consideration of the followingdetailed description of the invention taken in conjunction with theexamples which illustrate exemplary embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention comprises forming a polyester copolymer from the mixtureconsisting essentially of terephthalic acid (TA) or dimethylterephthalate (DMT), ethylene glycol, adipic acid, and pentaerythritolwith the adipic acid being added in the amount of about 1.3 to 3.2weight percent of the TA or DMT and the pentaerythritol being added inthe amount of between 175 and 700 ppm of the TA or DMT. Furthermore, theadipic acid and ethylene glycol can be added as an adipic acid/ethyleneglycol/bishydroxyethyl adipate solution such that the amount of theadipic acid remains in the range of 1.3 to 3.2 weight percent of the TAor DMT.

As is known to those familiar with the commercial production ofpolyester, the polyester polymer can be formed from a starting mixtureof terephthalic acid and ethylene glycol, or from dimethyl terephthalateand ethylene glycol. The polyester may be manufactured using a batchprocess or a continuous process. The reaction proceeds through the wellknown steps of esterification and condensation to form polyethyleneterephthalate, commonly referred to as polyester or PET. A number ofcatalysts or other additives have been found to be useful in promotingeither the esterification or condensation reactions, or in addingcertain properties to the polyester. For example, antimony compounds arecommonly used to catalyze the condensation reaction and inorganiccompounds such as titanium dioxide (TiO₂) are commonly added asdelustrants or for other similar purposes.

The polyester is formed as a viscous liquid which is forced through aspinneret head to form individual filaments; a process generallyreferred to in the art as "spinning". The spun filaments aresubsequently drawn, heat-set, crimped, dried and cut with theappropriate lubricating finishes added in a conventional manner. It willbe understood by those familiar with textile manufacturing in generaland synthetic fiber manufacture in particular that the word "spinning"has two connotations in the art, the first meaning being a term used todescribe the manufacture of fiber from a polymer melt, and the secondbeing the twisting of fibers together--natural, synthetic, orblended--to form spun yarn. Both terms will be used herein in theirconventional sense. The polyester copolymer of the present invention isproduced by previously described production methods for polyester, i.e.,esterification followed by polymerization via condensation. A batchprocess or continuous process may be employed, and catalysts and/orother typical additives may be employed. It will be understood that thepresence or absence of such other materials does not affect theessential techniques or results of the present invention, although theymay modify or enhance the polyester copolymer in the same desirablemanner as for polyester itself.

A batch process of the present invention starts with esterificationperformed at atmospheric pressure and at 180° to 220° C. The reactorwill be loaded with dimethyl terephthalate, ethylene glycol and acatalyst as is conventionally carried out in a customary batch polyesterprocess. After esterification is complete, the adipic acid andpentaerythritol are added. Other additives such as delustrants (TiO₂),thermostabilizers, optical brighteners and/or bluing agents, etc., maybe added at this initial polymerization stage. The polymerization stageis run at 280°-300° C. at a strong vacuum of 0.3 to 3.0 mm Hg pressure.The target intrinsic viscosity of the polymer is 0.5 to 0.65deciliters/gm. Alternatively, the batch process can be madeincorporating terephthalic acid, ethylene glycol, catalyst, adipic acidand pentaerythritol.

The above batch process can be run in a manner such that the adipic acidand/or pentaerythritol is loaded with the other raw materials at thebeginning of the esterification process. Furthermore, it is contemplatedfor a batch operation that some of the adipic acid and/orpentaerythritol may be added with the raw materials at the beginning ofthe esterification process, while the remaining additives are added atthe beginning of the polymerization stage.

Alternatively, a continuous process of the present invention starts withthe flow of raw materials including terephthalate acid (TA) and ethyleneglycol (EG) in a ratio of TA/EG of 1.1 to 1.4 mole ratio. The adipicacid and pentaerythritol may be added with the TA and EG, or they may beadded during, or immediately after the esterification phase of theprocess. Like the batch process, other additives and/or catalysts may befed into the reactor with TA and EG, as is customary with continuousoperations for polyester. In all the above processes, the adipic acidand pentaerythritol can be added as a solution of adipic acid andethylene glycol and BHEA.

In the primary esterification stage of the continuous process, thereactor is run at a pressure of 20 to 50 psi and a temperature of 240°to 260° C. In the conventional secondary esterification stage of thecontinuous process, the reactor is run at atmospheric pressure and at atemperature of 250° to 280° C. At the low polymerization stage, thereactor is run at a pressure of 15 to 50 mm Hg and at a temperature of265° to 285° C. At the final polymerization stage, the continuousreactor is operated at a pressure of 0.3 to 3.0 mm Hg and at atemperature of 275° to 305° C.

The heat-setting temperatures employed in a drawing process are raisedhigh enough to set the desired tensile properties in the copolymerfilament and to maintain the shrinkage of the copolymer filamentsubstantially the same as the shrinkage of the nonenhanced polyesterfilament. In this regard, heat-setting temperatures most preferred aregenerally greater than 120° C. and preferably between about 140° and240° C. In conventional processes, heat-setting temperatures greaterthan 150° C. cause the dyeability of the fiber to decrease belowacceptable levels for a product which is desirably atmosphericallydyeable. The enhancement of the fiber provided by the present invention,of course, also exhibits when the fiber is dyed under pressurizedconditions.

As set forth herein, the temperatures expressed for heat setting havebeen measured from the middle of the last heat-set roll and thencorrected for shell loss to give a reasonable approximation of thecontact temperature of the shell of the heated roll with which the fiberis in contact.

It is known that an increase in adipic acid during the production ofpolyester copolymers will increase the dyeability and lower the strengthof the fiber. Furthermore, it is also known that the use ofpentaerythritol in a polyester copolymer at low levels will increase thestrength of the fiber but can also diminish the dyeability thereof. Theuse of the present invention boosts the physical properties and thedyeability of the fiber to values greater than the separate values.Specifically, the tensiles of the fiber relative to control fiber atequivalent dyeability or improved dyeability are improved. These higherfiber tensiles have been demonstrated to translate into improved textileyarn strengths in various examples. Alternatively, and depending uponthe application desired for the resulting fiber, yarn or fabric, thepresent invention can be used to boost the dyeability of a given fiberwhile maintaining tensiles substantially equivalent to an unmodified orcontrolled fiber. Thus, the present invention provides a unique balanceof improved physical properties and also improved dyeability of thepolyester copolymer compared with other polyesters and polyestercopolymers.

EXAMPLE

Table 1 illustrates a number of the characteristics of staple fiberformed according to the present invention, using terephthalic acid andethylene glycol with 2.6 weight percent adipic acid based on TA and 580ppm pentaerythritol based on TA. The fiber is referenced as Example 1.The control PET fiber, Cl, was a nominal 1.0 dpf (denier per filament)polyester homopolymer formed under otherwise identical conditionsrelative to Example 1. The control PET/adipate copolymer fiber, C2, wasa nominal 1.0 dpf polyester copolymer formed under otherwise identicalconditions. The PET/polyethylene glycol copolymer C3 was produced usingsufficient polyethylene glycol to produce a copolymer having 2.5% byweight polyethylene glycol. The polyethylene glycol (PEG) had an averagemolecular weight of approximately 400 grams per mole. ThePET/pentaerythritol polymer, C4, was produced at 520 ppm pentaerythritolbased on TA.

The dyeability of the samples was measured against the dyeability(calibrated as 100.0) of commercially produced 1.0 dpf unenhancedpolyester fiber and corrected for dpf variations. The dying conditionset forth is atmospheric dyeing (ATM DYE) having the followingparameters: 50:1 liquor ratio; 8% Tanadel IM (Butyl Benzoate); 1 g/lDS-12 swetling dispersing agent commercially from Syborn Corporation,Wellford, S.C.; Acetic Acid - pH (4.5-5.0); 2% Disperse blue 27; 3°F./min rate of rise; and 60 mins @210° F.

                  TABLE 1                                                         ______________________________________                                        Samples                                                                              ATM DYE    MODULUS     TENACITY  HAS                                   ______________________________________                                        C1     100        4.07        5.48      10.2                                  C2     102        3.54        4.86      9.0                                   C3     120        2.79        5.00      11.9                                  C4      95        3.71        5.9       5.6                                   .sup.  1                                                                             112        4.37        5.58      9.7                                   ______________________________________                                         C1  Control unenhanced PET                                                    C2  Control PET enhanced with adipic acid                                     C3  Control PET enhanced with polyethylene glycol                             C4  Control PET enhanced with pentaerythritol                            

As used in Table 1, tenacity is the breaking load expressed as grams perdenier, the modulus is the strength at 10% elongation expressed in gramsper denier; elongation is the percentage increase in length the filamentcan undergo before breaking, the hot air shrinkage (HAS) is the percentdecrease in length of the filament when exposed to air at 400° F.;tenacity, modulus and elongation being determined in accordance withASTM D-3822 for tensile properties.

Comparison of the physical properties as found in Table 1 illustratesthe property advantages of the invention over prior art fibers. One ofthe goals of the present invention is to attain a polyester copolymerhaving an atmospheric dye of greater than 100% of an unenhanced controlfiber as previously defined, a modulus of greater than 3.00 g/denier, atenacity of between 5 and 7 g/denier, and a HAS of less than 10%.Example 1 meets all the foregoing criteria having an atmospheric dye of112%, a modulus of 4.37 g/denier, a tenacity of 5.58 g/denier, and a HASof 9.7%. This combination of properties is superior to that of theunenhanced PET homopolymer in control example C1, which has anatmospheric dye of 100% and a HAS of 10.2%. In Control example C2, itwas found that although the modulus and the HAS properties are met, thetenacity property was not met. Control example C3, has good atmosphericdye and tenacity but, it was found that the modulus was 2.79 g/denierand the HAS is higher than desirable. Control example C4 lacksdyeability characteristics having a dyeability inferior to that of thenonenhanced polyester C1.

The improvement in the filament strength as shown above relative tostandard polyesters and other polyester copolymers is expected to be akey factor in obtaining the highest possible rotor speeds in open-endspinning. Present developments indicate that rotor speeds of 100,000 rpmor greater may be available in the near future. In other spinningtechniques, such increased strength is similarly required. Ring spinningat present speeds of 20,000 rpm, jet spinning, and friction spinning allcall for fibers having improved physical characteristics. The technologyof the present invention is expected to provide good spinningefficiencies at such speeds while producing a product that remainsdyeable with dispersed dyes under atmospheric conditions, particularlywhen combined with selected low dpf fiber (eg. 1.5 dpf or less). Theadvantages of the invention, however, are not limited to any particularsize fiber.

Although the applicants do not wish to be bound by any particular theorythat they may have regarding the invention, it is recognized that manyof the copolymer's physical characteristics reflect a degree ofcrystallinity of a polymer. In the production of filament from thepolymer, if all other factors are held substantially constant, thetensiles of the filament may be lowered when additives, such as adipicacid or pentaerythritol are present. Copolymers particularly exhibitlower tensiles because it is believed that the added comonomersinterrupt the otherwise homogenous polymer and changes itscrystallinity.

Alternatively, dyeability is enhanced by certain additives preciselybecause the homogeneity of the polymer is physically interrupted, givinga dye molecule or a pigment a physical or chemical opportunity to enterthe polymer structure. Similarly, dyeability is discouraged whencrystallinity is increased because of the lack of potential reactionsites, and it is therefore discouraged by higher temperatureheat-setting.

Shrinkage is another variable which must be controlled in fibers andresulting fabrics. Shrinkage is increased by a lesser degree ofcrystallinity because the more amorphous regions, or the regions of thecopolymer or additive in the polymer chain, tend to collapse under heatto a greater extent than do the more oriented or homogenous portions ofthe polymer. Shrinkage is correspondingly decreased by a higher degreeof crystallinity. All variables being equal, desirable low shrinkageproperties tend to be competitive with desirable dyeability properties.

Another variable which is desirably controlled is the extent oforientation of the polymer. It is known to those familiar with thenature of polymers, that orientation refers to a somewhat orderedcondition in which the long polymeric molecules are in a greater degreeof linear relationship to one another, but are not in the lattice-siteand bonding relationships with one another that would define a crystallattice. All other factors remaining equal, increased orientation shortof crystallization tends to result in increased shrinkage, asapplication of heat tends to randomize the otherwise oriented molecules.The randomization tends to be reflected as a decrease in fiber length asthe linearly oriented molecules move into less linear relations with oneanother.

As is further known to those familiar with such a process, the drawingconditions under which the filament is initially formed are variablesother than the heat-setting temperature that controllably affect theorientation of the polymer, and therefore, a number of the propertieswhich relate to the orientation such as tensiles, dyeability, andshrinkage. As used herein, the drawing conditions include the drawratio, natural draw ratio and draw temperature. Draw ratio is defined asthe ratio of the final length in which the drawn filament is heat set tothe initial length of the filament prior to drawing. Other variablesaside, a greater draw ratio increases the orientation of the polymerforming the filament, thereby increasing the tensiles and the shrinkageof the resulting fiber, but decreasing the dyeability. A lower drawratio decreases the tensiles and shrinkage of the fiber and increasesthe dyeability.

The natural draw ratio for a fiber is the draw ratio at which the fiberwill no longer "neck". Alternatively, this can be expressed as theamount of draw required to end necking and begin strain hardening of adrawn fiber. As is known to those familiar with filament processes, whena filament is first drawn, it forms one or more drawn and undrawnportions in which the drawn portions are referred to as the "neck". Atthe natural draw ratio, however, the neck and undrawn portions disappearand the filament obtains a uniform cross section which then decreasesuniformly (rather than in necks and undrawn portions) as the fiber isdrawn further. The natural draw ratio reflects the degree of orientationof the undrawn spun fiber, with a lower natural draw ratio reflecting ahigher degree of orientation, and vice versa.

The natural draw ratio is measured by placing a length of spun tow intoclamps mounted on an Instron tensile tester, stretching the bundle untilbreak and measuring the resultant stress. Preferred natural draw ratiois from 100 to 130%.

Draw temperature is defined as that temperature of the drawing mediumused to induce the spun filament to yield. The medium is applied priorto heat setting. Typical examples of a medium include steam, liquid andheated rolls. The lower the temperature, the less crystallinity in theyarn and the converse holds.

These relationships hold true for polyester homopolymers, as well as forcopolymers such as the present invention, so that the draw ratio cangenerally be selected to give desired tensiles within a given rangedefined by the nature of the polymer or copolymer. The contribution ofthe invention is the ability to, in combination, increase thedyeability, as well as the tensile strength. In other words, prior tothe present invention, the tensile strength and dyeability of polyesterfilament always moved in inverse relationship to one another.

Results of the present invention are demonstrated by the data summarizedin Tables 2 and 3. This data was generated based on normal regressionanalysis of conducted experiments. Table 2 shows data including drawratio (% natural draw ratio), draw temperature, heat set temperature,dyeability, strength, elongation and hot air shrinkage for regularpolyester fibers (control) heat set at 130, 140 and 150° C. and fibersformed using 2.7 weight percent adipic acid based on TA and 580 ppmpentaerythritol based on TA heat set at 130°, 140° and 150° C. Table 3summarizes the relationship between varying amounts of the adipic acidand pentaerythritol added to the resulting fiber characteristics.

                  TABLE 2                                                         ______________________________________                                                       Heat Set Temperature (°C.)                              Property   Fiber     130      140     150                                     ______________________________________                                        ATM DYE    Controls  109      106     103                                                Examples  129      126     122                                     MODULUS    Controls  3.81     3.96    4.11                                               Examples  3.69     3.77    3.85                                    TENACITY   Controls  5.09     5.19    5.29                                               Examples  5.41     5.40    5.39                                    ELONGATION Controls  34       32      30                                                 Examples  5        48      45                                      HAS        Controls  10       9.5     9                                                  Examples  9        8       8                                       ______________________________________                                         ATM DYE = Atmospheric Dye                                                     HAS = Hot Air Shrinkage                                                       Draw Ratio (% Natural Draw Ratio) = 105%                                      Draw Temperature = 72° C.                                         

                  TABLE 3                                                         ______________________________________                                        Weight                                                                        % Adipic                                                                      Acid        Penta   Ten      Mod  HAS  ATM DYE                                ______________________________________                                        Control                                                                              0         0      4.84   3.92 5.5  100                                  1      2.2      410     5.33   4.55 4.7  104                                  2      2.6      485     5.09   4.37 4.8  102                                  3      3.1      580     5.25   4.61 5.3  107                                  ______________________________________                                         Draw Ratio (% natural draw ratio) = 115%                                      Draw Temperature 66° C.                                                Heat set temperature 200° C.                                           Penta = pentaerythritol ppm of TA                                        

Thus, it is apparent that there has been provided in accordance with theinvention, a polyester copolymer and a method of preparing the polyestercopolymer incorporating adipic acid and pentaerythritol that fullysatisfies the objects, aims and advantages as set forth above. While theinvention has been described in conjunction with specific embodimentsthereof, it is evident that many alternatives, modifications andvariations will be apparent to those skilled in the art in light of theforegoing description. Accordingly, it is intended to embrace all suchalternatives, modifications and variations that fall within the sphereand scope of the invention.

That which is claimed is:
 1. A process of producing a polyester filamentwhich has a superior combination of tensile, dyeability and shrinkageproperties which enhance the characteristics of fibers, yarns andfabrics made therefrom, the method consisting essentially of:forming apolyester copolymer from a mixture consisting essentially ofterephthalic acid (TA) or dimethyl terephthalate (DMT), ethylene glycol,adipic acid and pentaerythritol, with the adipic acid being added in anamount of from 1.3 to 3.2 weight percent of TA or DMT andpentaerythritol is present in the amount from about 175 to about 700 ppmby weight of TA or DMT; forming a filament from the copolymer; drawingthe copolymer filament; and heat setting the drawn filament.
 2. Aprocess according to claim 1 wherein the step of heat-setting the drawnfilament is done at a temperature from about 120° to about 240° C.
 3. Aprocess according to claim 1 wherein the step of drawing the copolymerfilament comprises drawing the copolymer filament between 100 and 130%of the natural draw ratio.
 4. A process according to claim 1 wherein theprocess of drawing the copolymer filament comprises drawing thecopolymer filament at a temperature from about 57° to 72° C.
 5. Aprocess of producing a polyester filaments which has a superiorcombination of tensile, dyeability and shrinkage properties whichenhance the characteristics of fibers, yarns and fabrics made therefrom,the method of consisting essentially offorming a polyester copolymerfrom a mixture consisting essentially of terephtalic acid (TA) ordimethyl terephthalate (DMT), and a solution of ehtylene glycol, adipicacid, bishydroxyethyl adipate (BHEA) and pentaerytritol, with the totaladipic acid being added in an amount from 1.3 to 3.2 weight percent ofthe TA or DMT and pentaerythritol is added in the amount from abut 175to about 700 ppm by weight of TA or DMT; forming a filament from thecopolymer; drawing the copolymer filament; and heat setting the drawnfilament such that the polyester filament has a modulus of greater than3.00 g/denier, a tenacity of between 5 and 7 g/denier and a hot airshrinkage of less than 10%.
 6. A process according to claim 5 whereinthe step of heat-setting the drawn copolymer filament occurs during thetemperature from about 120° to about 240° C.
 7. A process according toclaim 5 wherein the step of drawing the copolymer filament occurs atbetween 100 and 130% of the natural draw ratio.
 8. A process accordingto claim 5 wherein the process of drawing the copolymer filament occursat a temperature from about 57° to 72° C.